I just want to get you input on the following idea. Until i have a airspeed pressure sensor i have decided to not attempt to control speed using SOG from the GPS. Instead my idea is do define a fixed "cruising power" that gives my plane a safe cruising speed at level flight.
To control altitude i use the GPS altitude and set pitch angle to get to the correct altitude. My idea is simply to add or subtract power to the engine in relation to pitch angle to keep the speed within safe limits.
power= cruise_power + pitch_angle * power_constant
It's a simple approach, but i think it could work until i can measure correct airspeed. What do you think?